Both in the medical field and in certain industrial applications it is desirable to relay an image through a tube of substantial length, such as an endoscope. This is used to inspect and manipulate items of interest in locations which are not easily accessible by other means. In the medical field, the opportunity to reduce patient trauma and after care, by employing so-called Minimally Invasive techniques, is of major interest. One of the more important aids to manipulation, when doing so normally, is binocular vision. Each eye sees a slightly different perspective of the subject, which allows the precise determination of distance and depth.
An endoscope typically comprises a series of imaging and relay lenses, which transfer an image, usually wide angle, from the distal end of the instrument to the eye piece at the other end. Here, there is a pupil. What is meant by pupil is an effective aperture which may be a relayed image of a physical stop at any position in the optical system. Conveniently, the user may place his eye at a pupil of the system to observe the image. Often the eye's function is assumed by a small video camera suitably coupled to the optical system of the endoscope. The simplest way, conceptually, to provide two different perspective views of a subject trough an endoscope, is to duplicate the optical relay system, with both systems running side by side down the instrument. This approach is often adopted, but requires careful alignment of one system's optical axis to that of the other, in order to provide the correct correspondence between different points in each image. A second approach, which is also well known, is to subdivide the pupil of the system, either sequentially using one camera or by sending the two slightly different images, formed from light from the different portions of the pupil, to respectively different cameras, simultaneously.
In EP-A-0 577 268 (Dumbreck) an example of such apparatus is provided in which the exit pupil of a monocular viewing device such as an endoscope is divided into two halves and the light from each half is used to form an image seen from a slightly different view point to that provided by the other. As an aid to separating the light from the different halves of the pupil, the use of a two-part polarising plate is described in which, for example, one direction of polarisation is imparted to the light from the left half of the pupil, and an orthogonal direction is imparted to the light passing through the right half. A polarisation selective beam splitter, such as a Nicol prism, may then be used to separate these polarised components to form separate images for viewing by left and right eyes respectively.
The disadvantage of such an approach is that, whereas the majority of alignment problems are avoided, the stereopsis (difference in perspective) achieved is limited to that occurring within the dimensions of the pupil. This can have a diameter as small as 0.3 mm or less at the distal end, where the subject will typically be at a range of distances of up to 60 mm or more. The effective separation of the two halves of the pupil is of the order of 0.15 mm in the above example. For the normal level of stereopsis at 15 mm, a separation of at least 0.5 mm is desirable. The above identified prior art does not reveal how to increase the separation of the two view points beyond that which can be achieved within the overall dimensions of system's pupil. Additionally, where polarisation is used to aid in the separation of light rays corresponding to the different view points, the useful light for the formation of the image corresponding to each eye will in practice be reduced by more than a factor of two.
In Patent Application GB 9607089.1 (Street), an embodiment of endoscopic apparatus is disclosed which, for stereoscopic use, may conveniently include the use of a birefringent component for the provision of two points of view. This description develops the invention further and overcomes some of the attendant difficulties associated with its practical implementation. Unlike the limitations imposed by the apparatus of EP-A-0 577 268, a view point separation greater than the dimension of a single pupil may be achieved and light from the whole of this pupil can contribute to each of the images viewed.